Square battery

ABSTRACT

A battery case ( 1, 8 ), substantially square in cross section, has four lateral walls ( 5, 6 ) each of which is gradually arc-shaped to expand outwardly with a radius of curvature R 1.  The cross section is adapted to satisfy equation R 1 /r=4 to 20, where r is the distance between a midpoint (C 2 ) of each lateral wall ( 5, 6 ) and the center point (C 1 ) in the cross section of the battery case ( 1, 8 ) and R 1  is the radius of curvature. In addition, the battery case ( 8 ) is rounded at each corner with a radius of curvature of R 2,  and has a cross section that satisfies equation R 2 /r=0.3 to 0.8, where r is the distance between the midpoint (C 2 ) of each lateral wall ( 6 ) and the center point (C 1 ) in the cross section of the battery case ( 8 ) and R 2  is the radius of curvature of each corner.

TECHNICAL FIELD

The present invention relates to a sealed prismatic battery having powergeneration elements accommodated in a battery case having asubstantially prismatic cross section.

BACKGROUND ART

Recently, as a variety of portable electrical devices have beendeveloped, the development of batteries for supplying drive power to thedevices has been increasingly valued as a critical one of key devices.Among other things, compact rechargeable batteries such as nickel-metalhydride rechargeable batteries and lithium rechargeable batteries havebeen developed and increasingly grown in demand to be used recently inapplications such as a power supply for driving hybrid electric vehiclesin addition to cellular phones, notebook personal computers, or videocameras.

In recent years, electrical devices such as cellular phones have beenincreasingly demanded to be more compact and thinner, which has been inturn raising requirements for more compact and thinner batteries astheir power sources. The present batteries are largely divided intocylindrical and prismatic ones in shape. For the cylindrical battery, aplurality of batteries may be accommodated in a pack case to form abattery pack or a plurality of batteries may be accommodated in thebattery storage portion of an electrical device. In either case, poorspace efficiency is provided due to the presence of many useless spacesor dead spaces as well as instability is caused inside the storagespace. This makes the cylindrical battery unsuitable for theaforementioned electrical device that are to be made thinner and morecompact. In contrast to this, the prismatic battery provides high spaceefficiency and can be accommodated with stability in the storage space,thus being suitable for the electrical device that are to be madethinner and more compact.

However, the sealed prismatic rechargeable battery may be subjected toan increase in pressure inside the battery due to gases produced duringcharge and discharge, and expansion of the power generation elementsaccommodated in the battery case. In this case, the battery case isdeformed to take a more stable shape, that is, a circular shape, suchthat long lateral walls of the battery case are subjected to arelatively large deformation to expand outwardly. Such a deformation ofthe battery case causes various deleterious effects. For example, in asealed battery where the outermost electrode plate of a spiral-woundelectrode group is in electrical contact with the inner surface of thebattery case, the contact resistance between the outermost electrodeplate and the battery case is increased, resulting in an increasedinternal resistance of the battery.

In this context, conventionally suggested are the following methods ofproviding means for preventing the battery case from being deformed dueto an increase in pressure inside a prismatic battery. Japanese PatentLaid-Open Publication No. Sho. 62-93854 discloses providing part of abattery case with a portion having an increased thickness, therebyreinforcing the battery case. Japanese Patent Laid-Open Publication No.Hei. 7-326331 discloses a battery case having a rectangular crosssection, configured such that the inner surface of each corner isgradually arc-shaped to make the corners thicker than the long and shortlateral walls, which are connected to each other at each corner.

By providing part of the battery case with a portion having an increasedthickness, the battery case can be prevented from being deformed due toan increase in pressure inside the battery. However, the battery case isincreased in outer shape and thus cannot be made compact. On the otherhand, by increasing the thickness at the corners of the rectangularbattery case, the battery case can be prevented from being deformed dueto an increase in pressure inside the battery while preventing thebattery case from increasing in outer shape. However, the battery caseis reduced in volume and thus filled with a reduced amount of activesubstances for positive and negative electrodes of the electrode group,thereby reducing the energy density per volume.

The present invention has been developed in light of the aforementionedproblems. An object of the present invention is to provide a prismaticbattery which has a battery case that causes neither an increase inouter shape nor a decrease in inner volume but has a high pressureresistance enough to prevent the battery case from being deformed due toan increase in pressure inside the battery.

DISCLOSURE OF THE INVENTION

To achieve the above object, the present invention provides a prismaticbattery having a prismatic tubular battery case for housing an electrodegroup and electrolyte solution, with an opening of the battery casebeing sealed with a sealing member. The prismatic battery ischaracterized in that the battery case, substantially square in crosssection, has four lateral walls each being gradually arc-shaped to curveoutwardly with a radius of curvature R1, and in that the cross sectionsatisfies equation R1/r=4 to 20, where r is the distance between amidpoint of each lateral wall and the center point in the cross sectionof the battery case and R1 is the radius of curvature.

The prismatic battery is configured to have the battery casesubstantially square in cross section with each lateral wall of thebattery case being slightly expanded outwardly to satisfy that R1/r>4,and thus has high space efficiency generally equal to that of existingprismatic batteries. In addition, the prismatic battery is configuredsuch that the lateral walls of the battery case are gradually arc-shapedto expand outwardly to satisfy that R1/r<20. When an increase inpressure inside the battery occurs due to gases produced during chargeand discharge and expansion of the electrode group, this configurationpositively prevents each lateral wall from being deformed to furtherexpand outwardly. Moreover, since this prismatic battery increases itsinner volume by such an amount as provided by the lateral walls that areslightly expanded outwardly, it is possible to accommodate more powergeneration elements such as electrolyte solution in the increased space,thereby improving the energy density. In addition, with the battery casebeing entirely formed in a uniform thickness, the battery case is freefrom any decrease in inner volume. Even in generally the same outershape as that of an existing prismatic battery, the prismatic batteryprovides a large amount of current at the time of charge and discharge.This allows the prismatic battery to provide the battery characteristicsof a large capacity and a high energy density.

The invention also provides a prismatic battery having a prismatictubular battery case for housing an electrode group and electrolytesolution, with an opening of the battery case being sealed with asealing member. The prismatic battery is characterized in that thebattery case, substantially rectangular in cross section and with a pairof long lateral walls facing each other and a pair of short lateralwalls facing each other, has the cross section such that the longlateral walls are each gradually arc-shaped to curve outwardly with aradius of curvature R11 to satisfy equation R11/r1=4 to 20, where r1 isthe distance between a midpoint of the inner surface of the long lateralwall and the center point in the cross section of the battery case andR11 is the radius of curvature. The prismatic battery is alsocharacterized in that the short lateral walls are each graduallyarc-shaped to curve outwardly with a radius of curvature R12 to satisfyequation R12/r2=4 to 20, where r2 is the distance between a midpoint ofthe short lateral wall and the center point in the cross section of thebattery case and R12 is the radius of curvature.

The prismatic battery is configured to have the battery casesubstantially rectangular in cross section with the long lateral wallsof the battery case being slightly expanded outwardly to satisfy thatR11/r1>4 and with the short lateral walls being slightly expandedoutwardly to satisfy that R12/r2>4, and thus has high space efficiencygenerally equal to that of existing prismatic batteries. In addition,the prismatic battery is configured such that the long lateral walls ofthe battery case are gradually arc-shaped to expand outwardly to satisfythat R11/r1<20 and the short lateral walls are gradually arc-shaped toexpand outwardly to satisfy that R12/r2<20. When an increase in pressureinside the battery occurs due to gases produced during charge anddischarge and expansion of the electrode group, this configurationpositively prevents each lateral wall from being deformed to furtherexpand outwardly. Moreover, since this prismatic battery increases itsinner volume by such an amount as provided by the lateral walls that areslightly expanded outwardly, it is possible to accommodate more powergeneration elements such as electrolyte solution in the increased space,thereby improving the energy density. In addition, with the battery casebeing entirely formed in a uniform thickness, the battery case is freefrom any decrease in inner volume. Even in generally the same outershape as that of an existing prismatic battery, the prismatic batteryprovides a large amount of current at the time of charge and discharge.This allows the prismatic battery to provide the battery characteristicsof a large capacity and a high energy density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a prismatic battery accordingto a first embodiment of the present invention;

FIG. 2 is a cross sectional view taken along the line II—II of FIG. 1;

FIG. 3 is a cross sectional view illustrating a prismatic batteryaccording to a second embodiment of the invention;

FIG. 4 is a perspective view illustrating a prismatic battery accordingto a third embodiment of the invention;

FIG. 5 is a cross sectional view taken along the line V—V of FIG. 4; and

FIGS. 6A to 6C are perspective views illustrating the fabricationprocess of the prismatic battery in sequence.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, preferred embodiments of the present invention will be describedwith reference to the accompanying drawings. FIG. 1 is a perspectiveview illustrating a prismatic battery according to a first embodiment ofthe present invention. FIG. 2 is a cross-sectional view taken along theline II—II of FIG. 1. The prismatic battery is configured to have abottomed battery case 1 that is formed in the shape of a prismatic tubeto be substantially square in cross section. Inside the battery case 1,a spiral-wound electrode group 2 is accommodated which is made up of astrip of known positive electrode and negative electrode plates with aseparator interposed therebetween. An electrolyte solution (not shown)is then poured in the battery case 1, followed by fitting a sealingmember 4 into a cylindrical open end portion 3 of the battery case 1 andthen inwardly caulking the open peripheral edge of the open end portion3 to seal the portion 3 with the sealing member 4.

As shown in FIG. 2, the battery case 1, substantially square in crosssection, is configured to have four lateral walls 5 each of which isgradually arc-shaped to curve outwardly with a predetermined radius ofcurvature R1. The cross section is adapted to satisfy equation R1/r=4 to20, where R1 is the aforementioned radius of curvature and r is thedistance between a midpoint C2 of the inner surface of each lateral wall5 and the center point C1 in the cross section of the battery case 1.

The electrode group 2 is configured to have a strip of the positiveelectrode plate, a strip of the negative electrode plate, and a strip ofthe separator, each being spirally wound in the shape of a cylinder.Then, the electrode group 2 is preferably compressed using a die to berectangular in cross section corresponding to the cross section of thebattery case 1 so as to be press fit into the battery case 1. Thisconfiguration allows the electrode group 2, when inserted into thebattery case 1, to slightly relax its tension and thus slightly deformto curve outwardly due to its restoring force to its original circularshape, thereby allowing the electrode group 2 to come into contact witheach of the lateral walls 5.

The prismatic battery includes the battery case 1 substantially squarein cross section with each lateral wall 5 being slightly expandedoutwardly, and thus has high space efficiency generally equal to that ofexisting prismatic batteries. However, setting such that R1/r<4 wouldreduce the space efficiency to the same order as that of existingcylindrical batteries.

In addition, the prismatic battery is configured such that the fourlateral walls 5 of the battery case 1 are each gradually arc-shaped toexpand outwardly. When an increase in pressure inside the battery occursdue to gases produced during charge and discharge and expansion of theelectrode group 2, this configuration positively prevents each lateralwall 5 of the battery case 1 from being deformed to further expandoutwardly, unlike the existing prismatic battery with four lateral wallseach having a straight flat surface. In this case, setting such thatR1/r>20 would make it impossible to sufficiently prevent the lateralwall 5 from expanding outwardly.

The prismatic battery significantly reduces the deformation of thebattery case 1 caused by the pressure inside the battery, therebymaintaining the contact resistance at a predetermined value between theoutermost electrode plate of the electrode group 2 and the innerperipheral surface of the battery case 1. This eliminates deleteriouseffect of increasing the internal resistance of the battery.Furthermore, when compared with the existing prismatic batteryrectangular in cross section, the prismatic battery increases its volumeby such an amount as provided by the four lateral walls 5 that areslightly expanded outwardly. This makes it possible to accommodate morepower generation elements such as electrolyte solution in the increasedspace, thereby improving the energy density. Furthermore, with thelateral walls 5 being entirely formed in a uniform thickness, thebattery case 1 prevents the lateral walls 5 from expanding without aportion increased in thickness being provided thereon as in theaforementioned conventional prismatic battery, thereby causing nodecrease in inner volume. This allows the prismatic battery to provide alarge amount of current at the time of charge and discharge, therebyproviding the battery characteristics of a large capacity and a highenergy density.

FIG. 3 is a cross-sectional view illustrating a prismatic batteryaccording to a second embodiment of the invention, where the samereference symbols designate the same or like elements as those of FIG.2. A battery case 7 of the prismatic battery is substantiallyrectangular in cross section and has a pair of long lateral walls 7 afacing each other and a pair of short lateral walls 7 b also facing eachother.

That is, the battery case 7 is configured such that the long lateralwalls 7 a are each gradually arc-shaped to curve outwardly with apredetermined radius of curvature R11, in order to satisfy equationR11/r1=4 to 20, where r1 is the distance between a midpoint C21 of theinner surface of a long lateral wall 7 a and the center point C3 in thecross section of the battery case 7 and R11 is the radius of curvature.The battery case 7 is also configured such that the short lateral walls7 b are each gradually arc-shaped to curve outwardly with apredetermined radius of curvature R12, in order to satisfy equationR12/r2=4 to 20, where r2 is the distance between a midpoint C22 of theinner surface of a short lateral wall 7 b and the center point C3 in thecross section of the battery case 7 and R12 is the radius of curvature.

Like the prismatic battery of the first embodiment, this prismaticbattery having the battery case 7 rectangular in cross sectionpositively prevents the battery case 7 from being deformed to furtherexpand outwardly when an increase in pressure inside the battery occursdue to gases produced during charge and discharge and expansion of theelectrode group 2. In addition, this prismatic battery accommodates morepower generation elements such as electrolyte solution than the existingprismatic battery, thereby making it possible to provide a large amountof current at the time of charge and discharge and provide the batterycharacteristics of a large capacity and a high energy density.

FIG. 4 is a perspective view illustrating a prismatic battery accordingto a third embodiment of the invention, and FIG. 5 is a cross sectionalview taken along the line V—V of FIG. 4, where the same referencesymbols designate the same or like elements as those of FIGS. 1 and 2.This prismatic battery is configured to have a battery case 8 that issubstantially square in cross section like the one according to thefirst embodiment. Inside the battery case 8, the spiral-wound electrodegroup 2 is accommodated which is made up of a strip of known positiveelectrode and negative electrode plates with a separator interposedtherebetween. An electrolyte solution is then poured in the battery case8, followed by fitting the sealing member 4 into a cylindrical open endportion 9 of the battery case 8 and then inwardly caulking the openperipheral edge of the open end portion 9 to seal the portion 9 with thesealing member 4.

As shown in FIG. 5, the battery case 8, substantially square in crosssection, is configured to have four lateral walls 6 each of which isgradually arc-shaped to curve outwardly with a predetermined radius ofcurvature R1. The cross section is adapted to satisfy equation R1/r=4 to20, where r is the distance between a midpoint C2 of the inner surfaceof each lateral wall 6 and the center point C1 in the cross section ofthe battery case 8 and R1 is the radius of curvature. This shape itselfis the same as that of the battery case 1 according to the firstembodiment. In addition to this shape, the four corners, each of whichis a boundary between two neighboring lateral walls 6, are eacharc-shaped with a predetermined radius of curvature R2. The arc shape isadapted to have a cross section that satisfies equation R2/r=0.3 to 0.8,where r is the distance between the midpoint C2 of the inner surface ofeach of the lateral walls 6 and the center point C1 in the cross sectionof the battery case 8 and R2 is the radius of curvature.

Thus, this prismatic battery provides the same effect as that of thefirst embodiment. In addition, suppose that a plurality of batteries areaccommodated side by side in a pack case to form a battery pack oraccommodated in the battery storage portion of an electrical device. Inthis case, since the prismatic battery is adapted to satisfy thatR2/r>0.3, the batteries prevent damage or scratches which may occur whenbrought into contact with another, and are never reduced in the effectof preventing an expansion of the lateral walls 6 due to an increase inpressure inside the battery. Furthermore, since it is set that R2/r<0.8,the space efficiency is never reduced.

FIGS. 6A to 6C are perspective views of the fabrication process of theprismatic battery in sequence, and by way of example, illustrate thefabrication process of the prismatic battery according to the thirdembodiment. The prismatic battery according to each of the first andsecond embodiments can also be fabricated in the same process. First, asshown in FIG. 6A, the battery case 8 is formed in the shape of abottomed cylinder to be substantially square in cross section as shownin FIG. 5. The electrode group 2 is configured such that the positiveelectrode plate and the negative electrode plate, both having the shapeof a strip, are superimposed on the other and spirally wound with aseparator interposed therebetween, and then compressed to besubstantially square in cross section corresponding to the cross sectionof the battery case 8. The electrode group 2 includes a positiveelectrode lead plate 11 that is protruded upwardly and a negativeelectrode lead plate (not shown) that is protruded downwardly. Theelectrode group 2 thus configured is inserted into the battery case 8.

After the electrode group 2 has been inserted into the battery case 8, awelding electrode bar is inserted into a hole at the center of theelectrode group 2 to attach the negative electrode lead plate to thebottom of the battery case 8 by resistance welding. Then, an insulatingplate 10 is placed on the upper end surface of the electrode group 2.For purposes of illustration, FIG. 6A illustrates the insulating plate10 that is placed on the upper end surface of the electrode group 2before the insertion into the battery case 8.

Subsequently, the battery case 8 is compressed with a die at a portionclose to the open end, thereby forming the cylindrical open end portion9 as shown in FIG. 6B. Additionally, while the battery case 8 is beingrotated, a roll die (not shown) being rotated in the direction oppositeto the rotational direction of the battery case 8 is pressed against theside surface of the open end portion 9 to form annular groove 12 thereonas shown in FIG. 4. The electrode group 2 is thereby fixed inside thebattery case 8.

Subsequently, a predetermined amount of electrolyte solution is pouredinto the battery case 8 through the hole at the center of the electrodegroup 2. Furthermore, the sealing member 4 shown in FIG. 6C is connectedby resistance welding at its filter (not shown) with the positiveelectrode lead plate 11. After an insulating ring (not shown) isinserted into the open end portion 9, the sealing member 4 is insertedinto the open end portion 9, thereby placed on an annular supportingportion expanded into the battery case 8, that are provided by theannular groove 12. With this arrangement, the open end portion 9 iscaulked inwardly to seal the portion 9 with the sealing member 4,thereby forming the prismatic battery shown in FIG. 4.

Now, the results of the experiments conducted by the present inventorsare described below. First, the prismatic battery according to the firstembodiment was fabricated as follows. That is, a strip of the positiveelectrode plate chiefly composed of nickel hydroxide and a strip of thenegative electrode plate chiefly composed of hydrogen-absorption alloy,between which interposed was a separator, were superimposed on theother. In addition, the positive electrode lead plate 11 was protrudedin one direction from the end of the positive electrode plate and thenegative electrode lead plate was protruded in the other direction fromthe end of the negative electrode plate. With this arrangement, thepositive electrode plate, the negative electrode plate, and theseparator were wound in a spiral manner to form the electrode group 2.The battery case 1 was configured to have a cross section to satisfythat R1/r=4.5, where the radius of curvature R1 was set to 50 mm and thedistance r was set to 11 mm, respectively. Using the electrode group 2and the battery case 1, a first prismatic battery was fabricated throughthe same process as described with reference to FIGS. 6A to 6C.

To fabricate the prismatic battery according to the third embodiment,the same electrode group 2 was constructed as that used for theaforementioned first prismatic battery. The battery case 8 wasconfigured to have a cross section to satisfy that R1/r=9.1 andR2/r=0.36, where the radius of curvature R1 was set to 100 mm, thedistance r was set to 11 mm, and the radius of curvature R2 of a cornerwas set to 4 mm, respectively. Using the electrode group 2 and thebattery case 8, a second prismatic battery was fabricated through thesame process as described with reference to FIGS. 6A to 6C.

Additionally, as comparative examples, fabricated were a third, fourth,and fifth prismatic battery. As shown in the table below, the prismaticbatteries of these comparative examples were fabricated using batterycases that satisfied conditions of the present invention neither thatR1/r=4 to 20 nor that R2/r=0.3 to 0.8. The five types of battery caseseach have a uniform thickness of 0.26 mm at all the portions includingthe lateral wall portions and the corners. All the batteries arenickel-metal hydride batteries.

After having been subjected to an ambient temperature of 25° C. for 12hours, each of the prismatic battery was charged and discharged for thefirst time (charged for 15 hours at a current of 0.1C, while dischargedfor 4 hours at a current of 0.2C) to examine the degree of expansion ofthe battery case. The degree of the expansion was determined as follows.That is, on the outer surface of the battery case at the center in thelongitudinal direction, the distance between the centers of lateralwalls that were opposite to each other was measured before the charging.Then, after the charging was carried out until the internal pressure ofthe battery reached 15 kgf/cm² at which a safety vent was initiated inthe sealing member 4, the distance between the same points as mentionedabove was measured. Then, a value was determined by subtracting thedistance before the charging from that after the charging. The resultsobtained in the tests are shown in Table 1. In Table 1, the first tofifth batteries are shown by B1 to B5, the radius of curvature of alateral wall by R1, and the radius of curvature of a corner by R2,respectively. The numerical values are all expressed in mm except forR1/r and R2/r.

TABLE 1 Before After Degree of R1 R2 R1/r R2/r test test expansion B1 50— 4.5 — 22.00 22.95 0.95 B2 100 4 9.1 0.36 22.00 22.73 0.73 B3 — 4 —0.36 22.00 23.65 1.63 B4 250 — 22.7 — 22.00 23.60 1.60 B5 100 2 9.1 0.1822.00 23.55 1.55

As can be seen clearly from the test results, the first and secondprismatic batteries B1, B2 related to these tests have a degree ofexpansion of 0.95 mm and 0.73 mm, respectively, which are far less thanthat of the third, fourth, and fifth prismatic batteries of thecomparative examples. This is because the lateral walls areappropriately arc-shaped to expand outwardly within the range of R1/r=4to 20, thereby preventing expansion.

In contrast, the third prismatic battery B3 is rounded at the cornersbut is provided with the flat lateral walls, thereby making the degreeof expansion as much as 1.63 mm. The fourth prismatic battery B4 allowsthe lateral walls to expand outwardly with a large radius of curvature(R1=250 mm) under the condition of R1/r>20, thereby also resulting ininsufficient prevention of expansion to provide a degree of expansion asmuch as 1.60 mm. Furthermore, the fifth prismatic battery B5 isconfigured such that the lateral walls are gradually arc-shaped toexpand outwardly within the preferred range of 4<R1/r<20, however, thecorners are rounded such that R2/r<0.3. This reduces the effect ofpreventing expansion in half, thereby resulting in a degree of expansionof 1.55 mm that is greater than that of the first and second prismaticbatteries B1, B2. From those results described above, it has been foundthat the lateral walls should be expanded outwardly within the range ofR1/r=4 to 20 and each corner should be rounded within the range ofR2/r=0.3 to 0.8 in order to efficiently prevent expansion at the time ofan increase in pressure inside the battery.

Industrial Applicability

As described above, according to the prismatic battery of the presentinvention, the lateral walls of a battery case, which is substantiallyrectangular in cross section and has generally the same high spaceefficiency as that of an existing prismatic battery, are each graduallyarc-shaped to expand outwardly. This makes the battery case suitable forpositively preventing each lateral wall from being deformed outwardlywhen an increase in pressure inside the battery occurs due to gasesproduced during charge and discharge and expansion of an electrodegroup.

Furthermore, since this prismatic battery increases its inner volume bysuch an amount as provided by the lateral walls that are slightlyexpanded outwardly, it is possible to accommodate more power generationelements such as electrolyte solution in the increased space, therebyimproving the energy density. In addition, with the battery case beingentirely formed in a uniform thickness, it is possible to prevent thelateral walls from expanding. Thus, without any decrease in inner volumeeven in generally the same outer shape as that of an existing prismaticbattery, the prismatic battery provides a large amount of current at thetime of charge and discharge. This makes the prismatic battery usefulfor providing the battery characteristics of a large capacity and a highenergy density.

What is claimed is:
 1. A prismatic battery comprising a prismatictubular battery case (1, 8) for housing an electrode group (2) andelectrolyte solution, with an open end of the battery case being sealedwith a sealing member (4), wherein the battery case (1, 8), beingsubstantially square in cross section, has four lateral walls (5, 6)each being gradually arc-shaped to curve outwardly with a radius ofcurvature R1, and the cross section satisfies equation R1/r=4 to 20,where r is a distance between a midpoint (C2) of each lateral wall (5,6) and a center point (C1) in the cross section of the battery case (1,8) and R1 is the radius of curvature.
 2. The prismatic battery accordingto claim 1, wherein the battery case (8) is rounded at each corner witha radius of curvature of R2, and has a cross section satisfying equationR2/r=0.3 to 0.8, where r is the distance and R2 is the radius ofcurvature.
 3. The prismatic battery according to claim 1, wherein thecross section of the electrode group (2) is formed corresponding to thecross section of the battery case (1, 8).
 4. A prismatic batterycomprising a prismatic tubular battery case (7) for housing an electrodegroup (2) and electrolyte solution, with an open end of the battery case(7) being sealed with a sealing member (4), wherein the battery case(7), being substantially rectangular in cross section and with a pair oflong lateral walls (7 a) facing each other, has the cross section suchthat the long lateral walls (7 a) are each gradually arc-shaped to curveoutwardly with a radius of curvature R11 to satisfy equation R11/r1=4 to20, where r1 is a distance between a midpoint (C21) of the long lateralwall (7 a) and a center point (C3) in the cross section of the batterycase (7) and R11 is the radius of curvature, and the battery case (7),being substantially rectangular in cross section and with a pair ofshort lateral walls (7 b) facing each other, has the cross section suchthat the short lateral walls (7 b) are each gradually arc-shaped tocurve outwardly with a radius of curvature R12 to satisfy equationR12/r2=4 to 20, where r2 is a distance between a midpoint (C22) of theshort lateral wall (7 b) and the center point (C3) in the cross sectionof the battery case (7) and R12 is the radius of curvature.
 5. Theprismatic battery according to claim 4, wherein the cross section of theelectrode group (2) is formed corresponding to the cross section of thebattery case (7).